The present invention relates to the use of downlink macro-diversity in a digital radio communication network. More precisely the present invention relates to a method of modifying the transmitted power distribution used by base stations transmitting a signal to a mobile station on the macro-diversity principle and to a mobile station for receiving such a signal.
Downlink macro-diversity consists in a principle according to which a mobile station receives several copies of a same signal from a set of base stations, called active set. In this situation, the mobile station is supposed to combine all received signals in order to achieve the maximum possible diversity effect. Whatever a fading on one link between the mobile station and a base station, the connection doesn""t break thanks to the subsequent links between the mobile station and the other base stations.
Downlink macro diversity is especially used when a mobile station is in a soft handover state.
Usually, the total power to be transmitted to the mobile station is determined by a common entity controlling the base stations, according to power control procedures. The distribution of the total power among the base stations belonging to the active set corresponds simply to similar relative powers transmitted by all the base stations. Then if Ptot is the total power to be transmitted, Ptot/N will be the relative power transmitted by all the N base stations belonging to the active set.
This solution assumes that all base stations belonging to the active set are in similar conditions to reach the mobile station. Actually, in such a case, the optimum diversity effect is obtained when all base stations transmit a similar relative power to the mobile station. The quality of service experienced at the mobile station is optimum. However, when the base stations belonging to the active set experience different transmitting conditions to reach the mobile station, such a simple solution shows no more an optimum quality of service at the mobile station.
It is an object of the invention to provide a method to take into account the different transmitting conditions for the base stations to reach the mobile station and to optimize the quality of service for the signal received with diversity at the mobile station.
These and other objects of the invention are attained by a method of modifying a current transmitted power distribution used by at least two base stations transmitting a signal to a mobile station on the macro-diversity principle, said method comprising the steps of: (i) measuring, at said mobile station, a quantity related to the signal to interference ratio for the part of said signal transmitted by each base station; (ii) processing some values by weighting and summing said measured quantities, a set of weighting coefficients corresponding to one hypothetical transmitted power distribution supposed to have been used by said base stations in place of said current transmitted power distribution; (iii) associating to each of said values, a value of a quality of service according to a known function; and (iv) replacing said current transmitted power distribution by one of said hypothetical transmitted power distributions providing a better value of quality of service than said current transmitted power distribution.
In accordance with a further aspect of the invention, there is provided a mobile station belonging to a digital radio communication system, said mobile station being able to receive a signal from at least two base stations performing macro-diversity, a current transmitted power distribution being used by said base stations, said mobile station comprising means to measure a quantity related to the signal to interference ratio for the part of said signal transmitted by each base station, said mobile station being characterized in that it further comprises: means to process some values by weighting and summing said measured quantities, a set of coefficients corresponding to one hypothetical transmitted power distribution supposed to have been used by said base stations in place of said current transmitted power distribution; means to associate to each of said values a value of a quality of service according to a known function; means to select a new transmitted power distribution among said hypothetical transmitted power distributions providing a better value of quality of service than said current transmitted power distribution; and means to send a request for said base stations to use the new transmitted power distribution.
Further advantageous features of the invention will be apparent from the description below and the appended claims.